Climatologists have recognized for decades that urban areas are warmer than rural areas. This is familiar to all of us; how many times have you heard a weather forecast that included, “possible frost in outlying areas?” The fact that concentrations of people, automobiles, buildings and factories create warm zones has long been called the Urban Heat Island Effect. Measuring the UHIE is very difficult, but about the fact that it exists, there is no dispute.
Thus, it is astonishing that the U.N.’s IPPC and other global warming advocates have published purported data on global temperatures that entirely fail to deal with the UHI phenomenon. This failure is especially glaring in view of the fact that 1) the entire global warming hysteria is predicated on post-1970s warming, while 2) there are three sources of global temperature data–satellite, ocean, and ground-based–and only the earth data show any warming trend. Scientists have long recognized that the rising temperatures measured at the world’s many weather stations are partly, and maybe mostly, the result of the increasing urbanization of the areas that surround those stations. But quantifying the UHIE is complicated at best.
At Watts Up With That, Dr. Roy Spencer outlines his analysis, which tries to quantify the magnitude of the Urban Heat Island Effect. He notes that the “gold standard” for earth (as opposed to satellite or ocean) temperature data is the data produced by the University of East Anglia, the CRUTem3 dataset. (Yes, these are the same data that were the subject of the original Climategate email scandal.) Spencer compared temperature trends as reported in the CRUTem3 dataset for matched weather stations in nearby areas with differing levels of population density. His approach seems to have been quite rigorous. His conclusion:
The CRUTem3 temperature linear trend is about 15% warmer than the lowest population class temperature trend. But if we extrapolate the results in the first plot above to near-zero population density (0.1 persons per sq. km), we get a 30% overestimate of temperature trends from CRUTem3.
If I increase the number of population classes from 3 to 5, the CRUTem3 trend is overestimated by 60% at 0.1 persons per sq. km, but the number of grids which have stations representing all 5 population classes averages only 10 to 15 per month, instead of 100 per month. So, I suspect those results are less reliable.
I find the above results to be quite compelling evidence for what Anthony Watts, Pat Michaels, Ross McKitrick, et al., have been emphasizing for years: that poor thermometer siting has likely led to spurious warming trends, which has then inflated the official IPCC estimates of warming. These results are roughly consistent with the McKitrick and Michaels (2007) study which suggested as much as 50% of the reported surface warming since 1980 could be spurious.
Spencer did a separate analysis of the data for the United States, probably the most complete and reliable set of data available to climatologists. The results were even more striking:
UPDATE: I’ve appended the results for the U.S. only, which shows evidence that CRUTem3 has overstated U.S. warming trends during 1973-2011 by at least 50%.
I’ve computed results for just the United States, and these are a little more specific. The ISH stations were once again stratified by local population density. Temperature trends were computed for each station individually, and the upper and lower 5% trend ‘outliers’ in each of the 3 population classes were excluded from the analysis. For each population class, I also computed the ‘official’ CRUTem3 trends, and averaged those just like I averaged the ISH station data.
The results in the following plot show that for the 87 stations in the lowest population class, the average CRUTem3 temperature trend was 57% warmer than the trend computed from the ISH station data.
So based on this analysis, around one-half of the alleged warming claimed by the IPCC is actually the result of the disproportionate location of weather stations in areas that are becoming more urbanized.
There are many more problems with the data on which global warming hysteria rests; for example, those who have inspected weather stations in the U.S. have reported that well over half are out of compliance with standards for such stations, e.g. because they are next to heating vents.
Global warming hysteria is politics, not science. If the IPCC enterprise were science, it would at least make a good faith effort to deal with major defects in the temperature data, like the UHIE. Alarmists refuse to address the UHIE not because they have a plausible case that it doesn’t exist–they don’t–but because there is an enormous amount of money being made by promoting “green” energy. You should read the comments to Dr. Spencer’s post; they draw out the difficulty of properly understanding and quantifying the urban heat island effect. But about the existence of the effect, and the fact that it substantially distorts the CRU/IPCC data, there is no doubt.